FIG. 7 shows a schematic diagram of a passive optical subscriber network of TDM (time division multiplexing) system, which connects a single station transceiver and a plurality of subscriber nodes.
A station transceiver 110 connects to a common port #C of optical multiplexer/demultiplexer 114 through an optical fiber 112. The optical multiplexer/demultiplexer 114 comprises an optical element to demultiplex an input light from the common port #C into N portions and output each demultiplexed light through ports #1˜#N, and to multiplex input light from each of the ports #1˜#N and output the multiplexed light through the common port #C. Each of the ports #1˜#N of the optical multiplexer/demultiplexer 114 connects to each of optical transceivers 118-1˜118-N belonging to respective subscribers #1˜#N via optical fibers 116-1˜116-N.
A TDM system is used for the communication between the station transceiver 110 and each of the optical transceivers 118-1˜118-N belonging to the subscribers #1˜#N respectively. Namely, each of the optical transceivers 118-1˜118-N extracts a signal in a timeslot assigned for itself out of the time division multiplexed signals (down signal) from the station transceiver 110, receives the extracted signal, and discards the rest of the optical signals in the other timeslots. Each of the optical transceivers 118-1˜118-N also outputs a signal to be transmitted for the station transceiver 110 onto the respective optical fibers 116-1˜116-N at timing according to its own assigned timeslot. The station transceiver 110 predeterminedly and -continuously synchronizes the station transceiver 110 and the optical transceivers 118-1˜118-N. By this operation, each of the optical transceivers 118-1˜118-N is able to know the timing of its assigned timeslot for transmission and reception. Description for the synchronizing procedure between the station transceiver 110 and the optical transceivers 118-1˜118-N is omitted here.
Used as a down signal to transmit from the station transceiver 110 to the optical transceivers 118-1˜118-N belonging to the respective subscribers #1˜#N and an up signal from the optical transceivers 118-1˜118-N belonging to the respective subscribers #1˜#N to the station transceiver 110 are optical carriers having a wavelength different from each other. In a conventional system, a 1.5 μm band optical carrier is used for the down signal and a 1.3 μm band optical carrier is used for the up signal.
The operation of a conventional system is explained below. The station transceiver 110 time-division-multiplexes a down optical signal Di (i=1˜N) destined for the respective subscribers #1˜#N and outputs the down optical signal Di onto the optical fiber 112. The down optical signal Di propagates on the optical fiber 112 and enters a common port #C of the optical multiplexer/demultiplexer 114. The optical multiplexer/demultiplexer 114 divides the time-division-multiplexed down optical signal Di into N portions and outputs each divided light onto the optical fibers 116-1˜116-N through the ports #1˜#N. Formatively, all the down optical signals D1-DN destined for the respective subscribers #1˜#N enter every one of the transceivers 118-1˜118-N. Each of the optical transceivers 118-1˜118-N extracts an optical signal in a timeslot assigned for itself out of the input optical signals, receives the extracted signal, and discards the rest of the optical signals in the other timeslots. For instance, the optical transceiver 118-1 exclusively receives a down optical signal D1, and the optical transceiver 118-2 exclusively receives a down optical signal D2.
Each of the optical transceivers 118-1˜118-N outputs an up optical signal Ui (i=1˜N) according to its own assigned timeslot onto the optical fibers 116-1˜116-N. The up optical signal Ui (i=1˜N) propagates on the optical fibers 116-1˜116-N and enters the ports #1˜#N of the optical multiplexer/demultiplexer 114 respectively. The optical multiplexer/demultiplexer 114 multiplexes the respective up optical signals Ui (i=1˜N) from the optical fibers 116-1˜116-N) and outputs the multiplexed up optical signal onto the optical fiber 112 through the common port #C.
When the optical transceivers 118-1˜118-N output the optical signal Ui (i=1˜N) onto the optical fibers 116-1˜116-N in the respective assigned appropriate timeslots, the up optical signals Ui on the optical fiber 112 are located on proper timeslots not overlapping each other in the time domain, as shown in FIG. 7. That is, the optical multiplexer/demultiplexer 114 multiplexes the respective up optical signals Ui without adjusting their time locations.
The up optical signal Ui being output onto the optical fibers 112 from the optical multiplexer/demultiplexer 114 transmits on the optical fiber 112 and enters the station transceiver 110. Since the station transceiver 110 synchronizes with each of the optical transceivers 118-1˜118-N, it can accurately separate each up optical signal Ui out of the input optical signals from the optical fiber 112.
In the above-described passive optical subscriber network, a plurality of subscribers shares one signal band in the time domain. Therefore, when one of the subscribers' units outputs an up optical signal in a timeslot other than the one assigned to itself due to some fault, the other subscriber's communication originally using the mistaken timeslot is inhibited.
For instance, supposing that the optical transceiver 118-1 outputs a continuous disturbance light onto the optical fiber 116-1, this disturbance light extremely deteriorates a signal-to-noise power ratio (SNR) of the up optical signals U2˜UN, which are output for the station transceiver 110 by the other optical transceivers 118-2˜118-N, on the optical fiber 112. This inhibits signal transmission from the optical transceivers 118-2-118-N to the station transceiver 110. This kind of situation can occur when a subscriber connects to a wrong communication device by mistake or a subscriber maliciously outputs an up optical signal that is not permitted.
When this type of fault occurs, it is most important to eliminate the fault factor as soon as possible. In particular, when the disturbance optical signal is transmitted continuously, all the subscribers' signals are interrupted and thus it is desirable to quickly solve the problem. However, conventionally, to identify the optical transceiver outputting the disturbance light out of all the optical transceivers 118-1˜118-N was impossible and thus there was no way but to check every transceiver one by one.